Energy transfer and charge separation dynamics in photoexcited pyrene-bodipy molecular dyads

An emissive charge-transfer excited-state at the well-defined hetero-nanostructure interface of an organic conjugated molecule and two-dimensional inorganic nanosheet

Precise engineering of excited-state interactions between an organic conjugated molecule and a two-dimensional semiconducting inorganic nanosheet, specifically the manipulation of charge-transfer excited (CTE) states, still remains a challenge for state-of-the-art photochemistry. Herein, we report a long-lived, highly emissive CTE state at structurally well-defined hetero-nanostructure interfaces of photoactive pyrene and two-dimensional MoS2 nanosheets via an N-benzylsuccinimide bridge (Py-Bn-MoS2). Spectroscopic measurements reveal that no charge-transfer state is formed in the ground state, but the locally-excited (LE) state of pyrene in Py-Bn-MoS2 efficiently generates an unusual emissive CTE state. Theoretical studies elucidate the interaction of MoS2 vacant orbitals with the pyrene LE state to form a CTE state that shows a distinct solvent dependence of the emission energy. This is the first example of organic-inorganic 2D hetero-nanostructures displaying mixed luminescence properties by an accurate design of the bridge structure, and therefore represents an important step in their applications for energy conversion and optoelectronic devices and sensors.

Stimulated Resonance Raman and Excited-State Dynamics in an Excitonically Coupled Bodipy Dimer: A Test for TD-DFT and the Polarizable Continuum Model

Bodipy is one of the most versatile and studied functional dyes due to its myriad applications and tunable spectral properties. One of the strategies to adjust their properties is the formation of Bodipy dimers and oligomers whose properties differ significantly from the corresponding monomer. Recently, we have developed a novel strategy for synthesizing α,α-ethylene-bridged Bodipy dimers; however, their excited-state dynamics was heretofore unknown. This work presents the ultrafast excited-state dynamics of a novel α,α-ethylene-bridge Bodipy dimer and its monomeric parent. The dimer's steady-state absorption and fluorescence suggest a Coulombic interaction between the monomeric units' transition dipole moments (TDMs), forming what is often termed a "J-dimer". The excited-state properties of the dimer were studied using molecular excitonic theory and time-dependent density functional theory (TD-DFT). We chose the M06 exchange-correlation functional (XCF) based on its ability to reproduce the experimental oscillator strength and resonance Raman spectra. Ultrafast laser spectroscopy reveals symmetry-breaking charge separation (SB-CS) in the dimer in polar solvents and the subsequent population of the charge-separated ion-pair state. The charge separation rate falls into the normal regime, while the charge recombination is in the inverted regime. Conversely, in nonpolar solvents, the charge separation is thermodynamically not feasible. In contrast, the monomer's excited-state dynamics shows no dependence on the solvent polarity. Furthermore, we found no evidence of significant structural rearrangement upon photoexcitation, regardless of the deactivation pathway. After an extensive analysis of the electronic transitions, we concluded that the solvent fluctuations in the local environment around the dimer create an asymmetry that drives and stabilizes the charge separation. This work sheds light on the charge-transfer process in this new set of molecular systems and how excited-state dynamics can be modeled by combining the experiment and theory.

Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs

ConspectusSymmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.

Chromophore Orientation-Dependent Photophysical Properties of Pyrene-Naphthalimide Compact Electron Donor-Acceptor Dyads: Electron Transfer and Intersystem Crossing

In order to study the effect of mutual orientation of the chromophores in compact electron donor-acceptor dyads on the spin-orbit charge transfer intersystem crossing (SOCT-ISC), we prepared naphthalimide (NI)-pyrene (Py) compact electron donor-acceptor dyads, in which pyrene acts as an electron donor and NI is an electron acceptor. The connection of the two units is at the 4-C and 3-C positions of the NI unit and the 1-position of the pyrene moiety for dyads NI-Py-1 and NI-Py-2, respectively. A charge transfer absorption band was observed for both dyads in the UV-vis absorption spectra. Upon nanosecond pulsed laser excitation, long-lived triplet states (lifetime is 220 μs) were observed and the triplet state was confined to the pyrene moiety. The ISC efficiency is moderate to high in nonpolar to polar solvents (singlet oxygen quantum yield: Φ_Δ = 14-52%). Ultrafast charge separation (ca. 0.81 ps) and charge recombination-induced ISC (∼3.0 ns) were observed by femtosecond transient absorption spectroscopy. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism; interestingly, the observed electron spin polarization pattern of the triplet state is chromophore orientation-dependent; and the population rates of the triplet sublevels of NI-Py-1 (P_x:P_y:P_z = 0.2:0.8:0) are drastically different from those of NI-Py-2 (P_x:P_y:P_z = 0:0:1).

Light-induced energy transfer followed by electron transfer in axially co-ordinated benzothiazole tethered zinc porphyrin-fullero[C60/C70]pyrrolidine triads

Benzothiazole (BTZ)-zinc porphyrin (ZnP) dyads, Dyad-1 and Dyad-2 connected together with two different spacers, ester and ethoxy esters, were synthesized and light induced energy and electron transfer events were investigated. Within these dyads, due to the spectral overlap of the BTZ emission with the ZnP absorption, a selective photoexcitation of BTZ at 325 nm resulted in the photo-induced energy transfer (PEnT) from 1BTZ* to ZnP displaying the quenching of the BTZ emission followed by the concurrent appearance of the ZnP emission at 600 and 650 nm suggesting the formation of the 1ZnP* [Formula: see text]. 1BTZ*-ZnP [Formula: see text] BTZ-1ZnP*. When the dyads are titrated with imidazole appended fullero[C[Formula: see text]/C[Formula: see text]]pyrrolidines, four supramolecular triads, involving the axial co-ordination of the imidazole to the zinc center of the ZnP, were formed and the assembly formation was systematically monitored by the optical absorption technique. Cyclic voltammetry and the density functional theory calculations have revealed that, in these triads, the zinc porphyrin acts as an electron donor and fullerene moiety as the electron acceptor. Steady state fluorescence studies revealed that, upon selective excitation of the ZnP moiety at 550 nm, the emission of ZnP at 600 and 650 nm was quenched revealing the occurrence of photo-induced electron transfer (PET) from 1ZnP* to fullerene moiety leading to the formation of charge separated state [Formula: see text]. BTZ-1ZnP* : (ImC[Formula: see text] BTZ-ZnP[Formula: see text]:(ImC[Formula: see text]. More importantly, when the supramolecular triads were excited at 325 nm, the wavelength at which the BTZ absorbs predominantly, the emission of the BTZ moiety which was quenched due to PEnT from 1BTZ* to ZnP followed by the PET from 1ZnP* to fullerene indicates the probability of occurrence of 1BTZ*-ZnP:(ImC[Formula: see text] [Formula: see text] BTZ-1ZnP*[Formula: see text]: (ImC[Formula: see text] BTZ-ZnP[Formula: see text]:(ImC[Formula: see text].

Fully Conjugated Pyrene-BODIPY and Pyrene-BODIPY-Ferrocene Dyads and Triads: Synthesis, Characterization, and Selective Noncovalent Interactions with Nanocarbon Materials

Several pyrene-boron-dipyrromethene (BODIPY) and pyrene-BODIPY-ferrocene derivatives with a fully conjugated pyrene fragment appended to the α-position(s) of the BODIPY core have been prepared by Knoevenagel condensation reaction and characterized by one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR), UV-vis, fluorescence spectroscopy, high-resolution mass spectrometry as well as X-ray crystallography. The redox properties of new donor-acceptor BODIPY dyads and triads were studied by electrochemical (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)) and spectroelectrochemical approaches. Formation of weakly bonded noncovalent complexes between the new pyrene-BODIPYs and nanocarbon materials (C₆₀, C₇₀, single-walled carbon nanotube (SWCNT), and graphene) was studied by UV-vis, steady-state fluorescent, and time-resolved transient absorption spectroscopy. UV-vis and fluorescent spectroscopy are indicative of the much stronger and selective interaction between new dyes and (6,5)-SWCNT as well as graphene compared to that of C₆₀ and C₇₀ fullerenes. In agreement with these data, transient absorption spectroscopy provided no evidence for any significant change in excited-state lifetime or photoinduced charge transfer between pyrene-BODIPYs and C₆₀ or C₇₀ fullerenes when the pyrene-BODIPY chromophores were excited into the lowest-energy singlet excited state. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations suggest that the pyrene fragments are fully conjugated into the π-system of BODIPY core, which correlates well with the experimental data.

Excitation/detection energy controlled anisotropy dynamics in asymmetrically cyano substituted tri-podal molecules

In the present work, the photophysical properties of two series of asymmetrical tri-podal molecules are studied, in order to determine the dependence of energy localization/delocalization phenomena on excitation and detection wavelength, by means of steady state, femtosecond time-resolved fluorescence and anisotropy spectroscopy. The molecules bear triphenylamine as an electron donating core and an acetylenic or olefinic π-conjugated bridge. At the periphery, they are substituted by no, one, two or three -CN groups used as electron acceptors. Thus, the compounds with only one or two -CN groups are asymmetrically substituted. As a comparison, the photophysics of their dipolar and quadrupolar analogues is also presented. The steady state absorption spectra of the asymmetrical tri-podal compounds exhibit a broadening and a low energy shoulder due to the splitting of the excited states. The fluorescence spectra are more red-shifted in the tri-podal molecules with a single -CN group, providing the first evidence of its mostly dipolar nature. Time-resolved anisotropy measurements by using different excitation and detection wavelengths provide clear evidence that the asymmetrical tri-podal molecules with one or two -CN groups behave like octupolar molecules upon high-energy excitation (the initial anisotropy is found 0.1-0.15), while upon low-energy excitation they reveal a behavior expected for linear dipolar or V-shaped quadrupolar molecules (the initial anisotropy is very close to 0.4 and 0.17, respectively). The symmetrical tri-podal compounds with no or three cyano groups, exhibit an anisotropy depolarization time of 2.5 ps attributed to energy hopping. The amplitude of this energy hopping component is wavelength dependent and increases as the excitation is shifted towards the long wavelength edge.

Using sulfur bridge oxidation to control electronic coupling and photochemistry in covalent anthracene dimers

For anthracene dimers bridged by a sulfur atom, modulating the sulfur oxidation state profoundly affects excited state behavior. The SO₂-bridge supports long-lived states and photodimerization, while the S-bridge undergoes intersystem crossing.

Covalently tethered bichromophores provide an ideal proving ground to develop strategies for controlling excited state behavior in chromophore assemblies. In this work, optical spectroscopy and electronic structure theory are combined to demonstrate that the oxidation state of a sulfur linker between anthracene chromophores gives control over not only the photophysics but also the photochemistry of the molecules. Altering the oxidation state of the sulfur linker does not change the geometry between chromophores, allowing electronic effects between chromophores to be isolated. Previously, we showed that excitonic states in sulfur-bridged terthiophene dimers were modulated by electronic screening of the sulfur lone pairs, but that the sulfur orbitals were not directly involved in these states. In the bridged anthracene dimers that are the subject of the current paper, the atomic orbitals of the unoxidized S linker can actively mix with the anthracene molecular orbitals to form new electronic states with enhanced charge transfer character, different excitonic coupling, and rapid (sub-nanosecond) intersystem crossing that depends on solvent polarity. However, the fully oxidized SO₂ bridge restores purely through-space electronic coupling between anthracene chromophores and inhibits intersystem crossing. Photoexcitation leads to either internal conversion on a sub-20 picosecond timescale, or to the creation of a long-lived emissive state that is the likely precursor of the intramolecular [4 + 4] photodimerization. These results illustrate how chemical modification of a single atom in the covalent bridge can dramatically alter not only the photophysics but also the photochemistry of molecules.

Ultrafast excitation energy transfer in a benzimidazole-naphthopyran donor-acceptor dyad

The excited-state dynamics of a donor-acceptor dyad composed of 1-propyl-2-pyridinyl-benzimidazole (PPBI) as donor and the photochromic molecular switch diphenylnaphthopyran (DPNP) as acceptor linked via an ester bridge has been investigated by a combination of static and time-resolved spectroscopies and quantum chemical calculations. The UV absorption spectrum of the dyad is virtually identical to the sum of the spectra of its individual constituents, indicating only weak electronic coupling between the donor and acceptor in the electronic ground state. After selective photoexcitation of the PPBI chromophore in the dyad at λpump = 310 nm, however, a fast electronic energy transfer (EET) from the donor to the acceptor is observed, by which the lifetime of the normally long-lived excited state of PPBI is reduced to a few ps. Enabled by the EET, the acceptor switches from its ring-closed naphtopyran form to its ring-opened merocyanine form. The singular value decomposition-based global analyses of the measured femtosecond time-resolved transient absorption spectra of the dyad and its two building blocks as reference compounds allowed us to determine a value for the EET time constant in the dyad of τ = 2.90 ± 0.60 ps. For comparison, Förster theory predicts characteristic FRET times between 1.2 ps ≤ τ ≤ 4.2 ps, in good agreement with the experimental result.

Dynamic exciton localisation in a pyrene-BODIPY-pyrene dye conjugate

The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.

Synthesis and aggregation properties of boron-dipyrromethene dyes conjugated with guanine units

Two boron-dipyrromethene dyes bearing a conjugated guanine unit (G-BODIPYs) 1 and 2 were synthesized and fully characterized. The self-assembly properties of these dyes were investigated by X-ray crystallography, [Formula: see text]H NMR and UV-vis spectroscopy. As revealed by X-ray crystal structure studies, G-BODIPY 1 self-assembled into ribbon-like structures due to the intermolecular hydrogen bonding and [Formula: see text]–[Formula: see text] stacking interaction. Concentration-dependent [Formula: see text]H NMR experiments confirmed the formation of hydrogen bonds of the guanine units in solution for both dye 1 and 2. In the presence of K[Formula: see text], the characteristic signals for the formation of cyclic G-quadruplex structures were observed in the [Formula: see text]H NMR study. Aggregation of G-BODIPY dyes was further monitored by UV-vis absorption spectroscopy by varying the solvent polarity and temperature. H-type aggregates of dye 1, which was characterized by a new hypsochromically shifted absorption band with [Formula: see text] 461 nm, was obtained. In the presence of K[Formula: see text], the enhancement of stability was observed for the H-aggregates of dye 1.